Sedimentary Features and Implications for the Precambrian Non-stromatolitic Carbonate Succession: A Case Study of the Mesoproterozoic Gaoyuzhuang Formation at the Qiangou Section in Yanqing County of Beijing

In the long Precambrian period,stromatolitic carbonate successions were very common. However,the non-stromatolitic carbonate succession that is marked by subtidal deposits shows a sharp contrast to the stromatolitic carbonate succession.Both the non-stromatolitic and the stromatolitic carbonate successions are important clues for the further understanding of the evolving carbonate world of the Precambrian.The Mesoproterozoic Gaoyuzhuang Formation at the Qiangou section in northwestern suburb of Beijing is a set of more than 1000 m-thick carbonate strata that can be divided into four members(or subformations),in which a non-stromatolitic carbonate succession marked by the scarcity of stromatolites makes up the third member of the formation.This non-stromatolitic carbonate succession can further be subdivided into three third-order sequences that are marked by the regular succession of sedimentary facies.In third-order sequences,a lot of subtidal carbonate meter-scale cycles made up of medium-bedded leiolite limestones and thin-bedded marls constitute their transgressive system tracts(TSTs)and the early high-stand system tracts(EHSTs),a lot of meter- scale cycles made up by thin-bedded limestones and marls constitute their condensed sections(CSs), and thick-bedded to massive dolomitic limestones or lime dolomites make up the late high-stand system tracts(LHSTs).The particularly non-stromatolitic carbonate succession making up the third member of the Mesoproterozoic Gaoyuzhuang Formation at the Qiangou section might be the representative of the non-stromatolitic carbonate succession of the Precambrian because of its special lithological features and particular sedimentary structures,and its general sedimentary features are helpful and meaningful for the further understanding of the evolution rules of the sophisticate and evolving carbonate world of the Precambrian.The time scale of the Gaoyuzhuang Formation is deduced as that from 1600 Ma to 1400 Ma;thus,the non-stromatolitic sedimentary succession making up the third member of the Gaoyuzhuang Formation may demonstrate a stromatolite decline event occurring at ca. 1450 Ma of the Proterozoic besides other three events that occurred respectively at ca.2000 Ma,ca. 1000 Ma and ca.675 Ma.The forming duration of this non-stromatolitic sedimentary succession of the third member of the Gaoyuzhuang Formation can be generally correlated with a similar sedimentary succession in North America,i.e.a non-stromatolitic sedimentary succession of the Helena Formation of the Belt Supergroup,which suggests that the stromatolite decline occurring at ca.1450 Ma might be a global event.Therefore,the non-stromatolitic sedimentary succession discussed in the paper provides an important example for further understanding of carbonate sedimentology in the Precambrian.